1. Field of the Invention
Cornea shape measurement is generally carried out to obtain the refractive power, the degree of astigmatism and the direction of the astigmatic axis of the cornea and is also used for the inspection of the base curve of contact lenses.
2. Description of the Prior Art
Heretofore, in the ophthalmometer or keratometer known from U.S. Pat. No. 1,750,931 and U.S. Pat. No. 3,932,030, an index has been projected onto the cornea of an eye to be examined, the size of the reflected image thereof from the cornea has been observed by the use of a microscope, and optical member such as a prism or the like has been moved and the refractive power, etc. of the cornea have been obtained from the amount of movement of the optical member.
However, this method has utilized observation by the eye and has therefore required much time to make the measurement. These factors have, in turn, led to a problem that an error can result from movement of the eye to be examined.
Now, generally, a cornea does not have a uniform curvature in the direction of each diametral line. Therefore, the image reflected by the cornea assumes an elliptical shape and to specify this ellipse, it is known to obtain the refractive power, etc. of the cornea by measuring the maximum values in the directions of three diametral lines. This operation utilizes the fact that when the central portion of the cornea is regarded as a toric surface, the variations of the curvature of the cornea in the directions of diametral lines can be regarded as being sine-wave-like. However, in the measurement in the directions of three diametral lines, it has been necessary to enhance the alignment accuracy in a plane perpendicular to the optical axis, namely, to make the measurement optical axis and the optical axis of the eye to be examined substantially coincident with each other. That is, it has been necessary to specify the center of the ellipse and thus, measurement has generally been difficult.